The practice of bleeding air from one or more gas turbine aircraft engines to provide pneumatic and thermal power to different aircraft systems is well known. The bleed air may be used for, for example, cabin pressurization and temperature control and other environmental control equipment. It may also be provided to thrust reversing systems, anti-icing equipment, and/or pneumatically powered equipment. Higher fuel efficiency and a reduction in engine wear are obtained by minimizing the amount of air that is bled.
Under normal operating conditions, it is desirable to distribute the burden of supplying air for these auxiliary functions among the several engines of an aircraft. Without accurate allocation of the airflow burden among the several engines, the engine having the greatest burden experiences disproportionately diminished fuel economy, elevated operating temperature, and increased wear.
Satisfactory flow balancing could be achieved if flow-control valves were provided in bleed air flow paths in addition to pressure regulating valves so that flow and pressure could each be regulated by a valve designed for one particular purpose. However, economic and weight considerations generally require that a single pressure regulation valve be used for both pressure and flow regulation.
One way of making a pressure control valve perform both functions is to superimpose a trimming signal based on flow rate information on a standard pressure regulation command which is issued to a pressure regulation valve. One disadvantage of this approach is that flow signal feedback can cause small amplitude, high frequency oscillations, or limit-cycling, of the pressure regulating valve, on the order of 0.5 to 2 Hz, for example. These oscillations may not significantly impair the function of the regulating system, but could lead to more rapid fatigue in valve linkages and accelerated wearing of the valve. Moreover, when a flow imbalance occurs, it is generally desirable to rapidly correct it; however, systems capable of such rapid corrections generally suffer from the above-mentioned limit cycling problem.
Occasionally, due to engine or cooling system malfunctions, heat exchangers that are often part of a bleed path may overheat and get damaged. Typical flow balancing systems are not designed to compensate for such damage.
It would therefore be desirable to provide a system and method for regulating pressure and flow rate in an engine bleed air system that provides accurate pressure and flow rate control and that avoids limit-cycling of the pressure regulating valves.